A. Field of the Invention
The present invention relates generally to instruments for measuring the permeability of a material, and, more particularly to system and method for measuring the permeability of a material.
B. Description of the Related Art
Many products or materials are provided with holes or perforations. Such products and materials require their permeability to be measured. Examples of such products and materials needing permeability measurements include: wallpaper; filters used for air, chemicals, etc.; materials affording the appropriate degree of liquid (ink, varnish, sizing) absorption in printing; porous bags and materials used in food packaging and agricultural fumigation; insulating materials; paper; textiles; etc.
One particular material provided with such holes or perforations are the wrappers of filter cigarettes or similar rod-shaped tobacco products. The perforations allow cool atmospheric air to enter the column of tobacco smoke. Such wrappers are called tipping paper. Running webs of tipping paper making up rod-shaped tobacco products may be perforated mechanically, electrically, or optically. For example, British Patent No. 1,588,980 discloses a perforating unit that employs a set of needles or analogous mechanical perforating tools that puncture selected portions of the running web. U.S. Pat. No. 2,528,158 and British Patent No. 1,604,467 disclose electro-perforating tools that employ heat-generating electrodes that combust selected portions of the running web. An optical perforating tool, as disclosed in U.S. Pat. No. 4,265,254, uses coherent radiation from a laser to make perforations of a desired size and with a high degree of reproducibility.
Conventional filter-tipped tobacco products are perforated in the region of their filter plugs to insure that atmospheric air can enter the column of tobacco smoke irrespective of the length of combusted portion of the tobacco-containing section of the product. It is desirable to regulate the permeability of wrappers of all articles of a given tobacco product in such a way that the permeability is consistent or deviates only negligibly from a predetermined value.
It is known to control perforations of tipping paper in response to permeability measurements, as discussed in U.S. Pat. Nos. 4,569,359, 4,121,595, 4,648,412 and 5,092,350. Known permeability measuring devices include pneumatic systems for measuring the pressure drop through the tipping paper. However, such pneumatic systems are frequently inaccurate and difficult to implement in a high volume production line where the web can travel through the perforator at speeds of 5000 to 6000 feet per minute.
Pneumatic measurements are frequently made off-line on a sample basis. In some conventional production lines, quality monitoring and control are accomplished through a combination of sampling and perforator adjustments. Initial setup can be accomplished by iterative trial and error in which the focus and power settings of the laser perforator are adjusted. After making tentative settings, the line is run to generate samples. The resulting samples are then tested in a pneumatic pressure drop instrument gauge. Once the desired operating results are achieved, a manufacturing inspector periodically samples the perforated product, for example, a sample could be taken of five foot sections of paper from the end of every third bobbin (or of every bobbin) to check for correct pressure drop. The paper could also be inspected by visual monitoring by holding the paper up to light to check generally for hole position and size. However, since such measurements are neither continuous nor in real time, defective perforation, if detected at all, would be determined after a large quantity of tipping paper has been perforated.
Optical monitoring devices for tipping paper perforation lines are also known, as discussed in U.S. Pat. Nos. 4,569,359 and 5,341,824. A conventional optical system for monitoring a perforation line is illustrated in FIG. 1 and described below. While such a system permits on-line monitoring of the process, in practice the output signal from this system has been found to correlate poorly with the pressure drops measured directly with pneumatic systems. Moreover, the system is affected by variations in the paper base sheet such as splices, extraneous holes, or thickness changes.
As shown in FIG. 1, the conventional optical monitoring system for monitoring perforations 102 in tipping paper 100 (traveling in direction 101) includes a light or optical source or sources 104 that shines a large circular area of light 106 onto the tipping paper 100. Typically, light source 104 is a halogen-based light source. Light 108 emanating through perforations 102 is received by a light or optical detector or detectors 110, and used to monitor and/or control the quality of the perforations 102 in tipping paper 100. The problem with such a conventional arrangement, as best shown in FIG. 3, is that the large circular area of light 106 has a diameter of about ten millimeters (mm) and illuminates an area having a number of perforations 102. Thus, the fine scanning and resolution capabilities of the conventional optical monitoring system are poor, reducing the reliability and accuracy of such a system.
Thus, there is a need in the art to provide a system and method for measuring the permeability of a material such as tipping paper that overcomes the problems of the related art.